dsh-ballast: DSH Web Context Window Attribution Plugin

`dsh-ballast` is a client plugin for DeepSeek Harness (DSH) Web, designed to address the challenges of context window management. It leverages the host's token meter to display, on a per-item basis, the current token usage of a surface, along with body summaries and pricing information. Core features include attributing token usage by message type, aggregated statistics, window pressure estimation, session list viewing, and cross-session top ranking. The plugin retrieves data through read-only interfaces, supports truncating body content after collapsing whitespace, and includes environment dependencies and applicable use cases, helping developers monitor context usage in real time and troubleshoot window overflow issues.